handbook of modern sensors

6.5 Optoelectronic Motion Detectors 247
object on the surface of one part of the sensor only, which is occupied by a single pair
of electrodes. The element generates a charge only across the electrode pair subjected
to a heat flux. When the thermal image moves from one electrode to another, the
current i flowing from the sensing element to the bias resistor R (Fig. 6.14B) changes
form zero, to positive, then to zero, to negative, and again to zero (Fig. 6.14A lower
portion). A JFET transistor Q is used as an impedance converter. The resistor R value
must be very high. For example, a typical alternate current generated by the element
in response to a moving person is on the order of 1 pA (10 −12 A). If a desirable output
voltage for a specific distance is v = 50 mV, according to Ohm’s law the resistor value
is R = v/i = 50 G (5 × 10 10 ). Such a resistor can not be directly connected to a
regular electronic circuit; hence, transistor Q serves as a voltage follower (the gain
is close to unity). Its typical output impedance is on the order of several kilohms.
Table A.9 lists several crystalline materials which possess a pyroelectric effect
and can be used for the fabrication of sensing elements. The most often used are the
ceramic elements, thanks to their low cost and ease of fabrication. The pyroelectric
coefficient of ceramics to some degree may be controlled by varying their porosity
(creating voids inside the sensor’s body).An interesting pyroelectric material is a polymer
film polyvinylidene fluoride (PVDF) which, although not as sensitive as most of
the solid-state crystals, has the advantages of being flexible and inexpensive. In addition,
it can be produced in any size and may be bent or folded in any desirable fashion.
In addition to the sensing element, an infrared motion detector needs a focusing
device. Some detectors employ parabolic mirrors, but the Fresnel plastic lenses (Section
4.6 of Chapter 4) become more and more popular because they are inexpensive,
may be curved to any desirable shape, and, in addition to focusing, act as windows,
protecting the interior of the detector from outside moisture and pollutants.
To illustrate how a plastic Fresnel lens and a PVDF film can work together, let us
look at the motion detector depicted in Fig. 6.15A. It uses a polyethylene multifaceted
curved lens and a curved PVDF film sensor [7]. The sensor design combines two
methods described earlier: a facet lens and a complex electrode shape. The lens and
the film are curved with the same radii of curvature equal to one-half of the focal
distance f , thus assuring that the film is always positioned in the focal plane of the
corresponding facet of the lens. The film has a pair of large interdigitized electrodes
which are connected to the positive and negative inputs of a differential amplifier
located in the electronic module. The amplifier rejects common-mode interference
and amplifies a thermally induced voltage. The side of the film facing the lens is coated
with an organic coating to improve its absorptivity in the far-infrared spectral range.
This design results in a fine resolution (detection of small displacement at a longer
distance) and a very small volume of the detector (Fig. 6.15B). Small detectors are
especially useful for the installation in devices where overall dimensions are critical.
For instance, one application is a light switch where the detector must be mounted
into the wall plate of a switch.
6.5.3.2 PIR Sensor Efficiency Analysis
Regardless of the type of optical device employed, all modern PIR detectors operate
on the same physical effect—pyroelectricity. To analyze the performance of such a